Eleven hijackers passed through Britain on their way to America without coming under surveillance, the government has admitted.

But Home Secretary David Blunkett said intelligence gathered about their movements in the UK would allow police to trace their associates.

Conservative leader, Iain Duncan Smith, has called for anti-terrorism laws to be changed to give intelligence services more flexibility in tracking terrorist suspects.

Meanwhile, Department of Transport officials met with airline industry representatives on Thursday to discuss possible new security measures, including armed guards on planes.

David Blunkett: Tracking terrorists' movements

A Downing Street spokesman said: "We have
given an undertaking to look across the range of different measures that could
be put in place."

He confirmed that putting air marshals on British planes is "one of the issues that will be looked at".

Mr Duncan Smith said existing anti-terrorism legislation had been undermined by the Convention of Human Rights, and should be made more enforceable.

Speaking on BBC Radio 4's Today programme, he said: "We need to tighten up some of the procedures, we need to give the intelligence resources greater capacity to follow people.

"But the British people need to know that this is about protecting them," he added.

Terror trail

Mr Blunkett said police were now working to pick up the trail left by the hijackers.

He said: "Some of them will have passed through, some will have stayed over.

"What we do now know is, having identified these people - because we do actually now have the line back to where they were - we can track not only their movements, but those who associated with them. That is the crucial issue."

This has transformed terrorism from a criminal justice issue to a
global security threat

Assistant Commissioner David Veness

Metropolitan Police Assistant Commissioner David Veness has warned that associates of the hijackers could attempt further terrorist attacks in the UK.

He said: "We have to consider the possibility of follow-up attacks.

"Clearly there is a need for new measures in what is a very serious change in the threat."

But the Home Secretary moved to ease public fears that terrorists could be operating in their midst.

Security alert

He told the BBC's Newsnight programme: "Of course we have to be vigilant. That is not the same as
believing that there is about to be an imminent attack in this country."

Asked if any of the terrorists' contacts might still be in the country, Mr
Blunkett said: "I believe that we shouldn't rule it out."

Scotland Yard has received more than 100 requests from the FBI to trace
suspects, witnesses and others connected with the case.

Members of the public have so far made 3,600 calls to an anti-terrorist hotline and police forces.

Mr Veness said: "I can confirm that we are pursuing reports in
respect of individuals who passed through the UK at British airports to
vigorously examine how long they were in the UK, in what identity, when they arrived and what they did between arrival and departure.

David Veness: Tough tactics

"The individuals could merely have
transited the UK and we must encompass the possibility, though we have nothing to substantiate it, that some of the preparation activity took place here."

Investigations are thought to be hampered by the use of multiple identities by the suspected hijackers.

As part of the early response to the new terrorist threat, the Special Branch
and the Anti-Terrorist Squad have been beefed up.

Former officers now serving elsewhere have been drafted back and security has been increased in financial districts. There are also plans for terrorism officers in every police force in England and Wales.

Mr Veness said the events in the US had "transformed terrorism from a criminal justice issue to a global security threat".

"We need to recognise the nature, the scale and the significance of this particular change is seismic."